5G: Breaking down the essentials of its Waveform and Multiple Access

Waveform and Multiple Access

When we dig deeper in to 5G we have to appreciate the deep, technical nuances that enable ultra-high speed, low latency wireless data transmission to tens or even hundreds of thousands of people within an given area. Without doubt, it’s an immense challenge.

5G Waveforms

There were many options for waveforms proposed at 3GPP, which were required to set performance targets to evaluate and compare each of them. The main proposals were compatible with many essential technologies, including MIMO, Spectral efficiency, Low Peak to Average Power ratio (PAPR), high time localization to support TDD systems and ultra-reliable low latency (URLLC) use cases, acceptable complexity and low out of band emissions.

For 3GPP Release 15 it was agreed that an OFDM-based waveform with Cyclic Prefix (CP) will be supported for both 5G NR download and upload. DFT-S-OFDM based waveform will be also supported, complementary to CP-OFDM waveform and used for an ehanced mobile broadband (eMBB) uplink up to at least 40GHz.

The CP-OFDM waveform can be used for either single-stream and multi-stream (i.e. MIMO) transmissions, while DFT-S-OFDM based waveform is limited to single stream transmissions by targeting scenarios with limited link budget.

Physical Channel Bandwidth.

The maximum channel bandwidth for 5G NR (Sub-6GHz) is 100MHz, while mmWave is a larger 400MHz. Compared to LTE, 5G NR is designed to have a higher bandwidth efficiency of up to 99%, versus up to 90% in LTE, where only 18 MHz in a 20 MHz channel was effectively used.

Numerologies

With an increasing number of use cases planned for 5G NR, a scalable and flexible physical layer design is required.

The main idea of OFDM is to divide a wide channel into orthogonal, narrow subcarriers. A set of parameters defines the division, which in turn expresses sub-carrier spacing, symbol length, cyclic prefix (CP) and transmission time interval (TTI). An application uses a fixed configuration of these parameters, which then receives a Numerology (a ‘number’). The different numerologies (configurations) are applied to different deployments (use types), that then provide different performance characteristics.

For example, a smaller sub-carrier spacing gives a larger the cell size, which is particularly suitable for lower frequency deployment. At the same time, larger sub carrier spacing allows for lower latency performance since the symbol duration will be shorter.

Carrier Bandwidth Part

The Carrier Bandwidth Part (CBP) concept was proposed by MediaTek engineers to 3GPP for the 5G standard. A carrier bandwidth part is a contiguous set of physical resource blocks, selected from a contiguous subset of the common resource blocks for a given numerology on a given carrier. It allows for the multiplexing of narrowband and wideband ‘user equipment’ (smartphones, tablets, vehicles etc), allowing for bandwidth adaptation that will afford device power savings. This is necessary for most modern devices where power efficiency is a key concern.

Frame Structure

These different numerologies will be then translated to several slots per sub-frame. The higher the sub-carrier spacing, the higher the number of slots per sub-frame. Here, new technologies were added to the frame structure definitions to deal with different numerologies, and the wide carrier bandwidth of 5G NR.

Modulation

For OFDM with cyclic prefix (CP), for both download and upload: QPSK, 16QAM, 64QAM, and 256QAM modulation methods are available.

For the DFT-s-OFDM with cyclic prefix (CP), for upload only: π/2-BPSK, QPSK, 16QAM, 64QAM and 256QAM modulation methods are available.

3GPP discussions are ongoing regarding support of 1024QAM.

Channel Coding

Low Density Parity Check (LDPC) coding is replacing the Turbo coding that was previously used for LTE data channels, while Polar Codes are replacing the Tail Biting Convolutional Codes (TBCC) used previously for LTE control channels, except for very small block lengths where repetition/block coding may be preferred.

MediaTek was among the first to do real 5G inter-operability testing with Polar Code for network capacity boost & low design complexity, in co-operation with Huawei.